At Sunesis Pharmaceuticals, we have been developing techniques for identifying and characterizing inhibitors of protein-protein interfaces. While there would be a large market for orally available drugs which target protein-protein complexes, standard methods of drug discovery have met with limited success. Using inhibition assays, scientists at Sunesis have generated lead compounds which inhibit cytokine receptor inter-actions. However, assays based on inhibition have limitations, and current challenges include (a) identifying the binding partner of the small molecules, and (b) developing assays which measure binding directly. Towards these ends, we propose to develop fluorescence resonance energy transfer (FRET) as a rapid assay for measuring the binding of small molecules to protein targets. In Phase I of this project, FRET will be used in a primary assay to screen compounds for binding to human growth hormone (hGH). Future work will investigate other targets and develop FRET as a secondary assay to further characterize the binding of lead antagonists of IL-4. Development of these spectroscopic assays will yield methods for rapidly identifying drug leads against protein-protein targets. PROPOSED COMMERCIAL APPLICATION: Validation of the FRET method will provide a new, general method for identifying small molecules for protein-protein or protein-ligand systems. FRET does not rely on inhibition of protein function and does not require a large fraction of protein binding to detect signal. Injectible hGH antagonists are currently in clinical trials for treatment of acromegaly and diabetic retinopathy. Anti-IL-4 antibodies are in clinical trials for treatment of asthma. Orally available small molecules would have benefits over injectable drugs.